• Journal of the Korea Concrete Institute
• /
• v.23 no.4
• /
• pp.471-478
• /
• 2011

For a practical simplicity in designing of reinforced concrete structures, the indirect crack controlling method of limiting bar spacing is adopted in KCI structural design provisions. In addition, a direct method for evaluating crack width is also provided in the appendix of the code. But there may be some mismatched results between these two crack controlling methods. In this study, limit values of maximum bar spacing calculated from KCI provisions, KCI appendix, and Frosch's equation are examined as concrete strength, cross-section height, and concrete cover are varied, and the differences are analyzed. From the results, it becomes clear that the differences between maximum bar spacing calculated from KCI code text provisions and those from KCI code appendix provisions are too significant to be neglected. Therefore, rational crack models are suggested in order to get rid of the discrepancy between the direct and indirect control methods.

• Journal of the Korea Academia-Industrial cooperation Society
• /
• v.19 no.5
• /
• pp.98-105
• /
• 2018

Cracks in RC members occurred as a result of material and structural factors. The crack width and a crack location are very difficult to examine. A direct crack control method and indirect crack control method to control a crack are presented in the KHBDC (LSD) and KSCDC (2012). In the KSCDC text, cracks are controlled by steel spacing indirectly under a service load. On the other hand, in the KSCDC appendix, cracks are controlled by a crack width directly under a sustained load. In particular, the loading state considered is different. On the other hand, cracks are controlled under a combination of service load and an effective elastic modulus is used in KHBDC. Therefore, in this study, an effective elastic modulus that can reflect the ratio of the sustained load and live load was applied, and a maximum steel spacing was calculated through a design crack width. A variable interpretation was carried out, and a rational crack control method was assessed. As a result, a steel spacing through the design crack width in the KSCDC was smaller than that from the design crack width in the KHBDC, which leads to a conservative design. In addition, the maximum steel spacing suggested in this study has a consistency eliminating the difference between direct crack control and indirect crack control.

• Proceedings of the KWS Conference
• /
• 2010.05a
• /
• pp.22-22
• /
• 2010

Eutectic composition phase with low melting point which solidifies at the final stage affects the solidification cracking at the intercellular or interdendritic area of welds and castings. If sufficient amount of eutectic composition liquid does not exist between the solidifying phases, the discontinuities remain as cracks. However, abundant amount of liquid eutectic composition existing in the final stage can flow into the discontinuities easily and heal the cracks. By flowing of liquid eutectic and healing of discontinuities, the possibility of cracking can be reduced when the amount of eutectic liquid is sufficient. For the solidification of pure metals, liquid eutectic does not exist and the interlocking of growing solid phases can be realized without interruption of liquid film. Therefore there is little possibility of solidification cracking in the case of welds and castings of pure metal. In a practical sense, the effective way to reduce or prevent the solidification cracking is making the composition of molten pool or melts near to the eutectic composition.

• Journal of the Korea Concrete Institute
• /
• v.22 no.5
• /
• pp.719-725
• /
• 2010

The analysis and experimental program of reinforced concrete (RC) structures for resistance against such extreme loads as earthquake, blast, and impact have been carried by many researchers and designers. Under the extreme loads, a large amount of energy is suddenly exerted to the structure, hence if the structure fails to absorb the impact energy, catastrophic collapse may occur. To prevent catastrophic collapse of structures, reinforced concrete must have adeguate toughness or it needs to be strengthened. The FRP strengthening method and SFRC are studied widely in resistance of impact load because of their high energy absorption capacity. In this study, drop weight impact tests were implemented to evaluate the impact resistance of SFRC and FRP strengthened RC beam while the total steel fiber volume fractions was fixed at 0.75% carbon FRP flexural strengthened RC beams. Futhermore, to prevent the shear-plug cracks when the impact load strikes the beams, additional FRP shear strengthening method are applied. The experimental, results showed that the FRP strengthened RC SFRC beams has high resistance of shear-plug cracks and crack width and SFRC has high resistance of concrete spalling failure compared to normal RC beams. The FRP flexural and shear strengthening RC beams has weakness in the spalling failure because the impact load concentrated the concrete face which is not strengthened with FRP sheets.

• Journal of the Korea Concrete Institute
• /
• v.24 no.3
• /
• pp.233-240
• /
• 2012

Hybrid SHCC is being researched actively for its excellent performance in controlling macro and micro cracks using macro and micro fibers, respectively. However, a significant autogenous shrinkage of SHCC is expected since it possesses high unit cement volume in its mix proportion, resulting in autogenous shrinkage cracks. Therefore, this study was performed to evaluate mechanical property of shrinkage-reducing type hybrid SHCC mixed together with steel fiber and PE fiber with excellent micro/macro crack controlling performance. In order to evaluate mechanical property of shrinkage-reducing type hybrid SHCC, replacement ratios of 0% and 10% of expansive admixture and water to binder ratios of 0.45, 0.3, and 0.2 were considered as variables. Then, shrinkage, compressive, flexural, and direct tensile tests were performed. The test results showed that mix proportion with W/B 0.3 significantly improved mechanical performance by using 10% replacement of expansive admixture.

• Journal of the Korea institute for structural maintenance and inspection
• /
• v.27 no.3
• /
• pp.30-37
• /
• 2023

The PSCB girder bridge is a closed cross-section in which the top and bottom flanges and the web are integrated, and the structural characteristics are generally different from the bridges in which the girder and the floor plate are separated, so a maintenance plan that reflects the characteristics of the PSCB girder bridge is required. As a result of analyzing damage types by collecting detailed safety diagnosis reports of highway PSCB girder bridges, most of the deterioration and damage occurring during use is concentrated on the top flange. In particular, cracks in the bridge direction on the underside of the top flange occurred in about 70 % of the PSCB girder bridges to be analyzed, and these cracks were judged to be caused by indirect loads such as heat of hydration and drying shrinkage rather than structural cracks caused by external loads. In order to improve durability and reduce maintenance costs of PSCB girder bridges in use, it is necessary to control restraint drying shrinkage cracks from the design stage. Therefore, in this paper, the cracks caused by drying shrinkage under restraint, which is the main cause of cracks under the flanges of the top part of the PSCB girder bridge, were directly calculated using the Gilbert Model, and the influencing factors such as the amount of reinforcing bars, diameter and spacing of reinforcing bars were analyzed. As a result of the analysis, it was found that the crack width caused by restraint drying shrinkage exceeded the allowable crack width of 0.2 mm for reinforcing bars with a reinforcing bar ratio of 0.01 or less based on the H16 reinforcing bar and a reinforcing bar with a diameter greater than H19 based on the reinforcing bar ratio of 0.01. Finally, based on the results of the crack width review, a method for controlling the crack width of the top flange of the PSCB girder bridge was proposed.

• Journal of the Korea Concrete Institute
• /
• v.20 no.4
• /
• pp.469-475
• /
• 2008

It is obvious that chloride penetration through cracks can threaten the durability of concrete substantially, according to the previous studies of author. It was proposed that crack depth corrseponded with critical crack width from the surface is a crucial factor in view of durability design of concrete structures. It is now necessary to deal with chloride penetration through microcracks characterized with the mixing features of concrete. The purpose of this study is examining the effect of mix proportional features of concrete such as coarse aggregate, high strengtherize of concrete and reinforcement of steel fiber on chloride penetration through cracks. Although small size of coarse aggregate can lead to many microcracks in concrete, the cracks should not impact on chloride penetration directly. On the contrary, chloride should penetrate through cracks easily in concrete with a large size of coarse aggregate because mixrocracks are connected to each other. Second, high strength concrete has an excellent performance to resist with chloride penetration. However, for cracked high strength concrete, its performance is reduced upto the level of ordinary concrete. Finally, steel fiber reinforcement is effective to reduce chloride penetration through cracks because steel fiber reinforcement can lead to reduce crack depth significantly.

• Proceedings of the Korean Geotechical Society Conference
• /
• 2009.03a
• /
• pp.1026-1031
• /
• 2009

21세기 대한민국은 지반분야에 있어서 과거 건설위주의 발전에서 유지관리의 중요성이 대두되는 시대로 변모하였다. 특히 이상기후에 의한 집중호우 발생이 빈번해진 상황에서 외부조건에 직접적으로 노출되어있는 사면구조물은 타 지반구조물보다 상대적으로 관리의 필요성이 더 높다. 사면의 유지관리 방법 중에서도 계측에 의한 상시 모니터링 시스템 구축 관련 기술이 주목을 받고 있으며, 장기적인 시스템 운용 측면에서 내구성이 우수한 광섬유 재료를 이용한 계측 시스템 개발이 국가적 지원을 받으며 연구가 활발히 진행 중에 있다. 본 연구는 암반사면의 균열부에 설치하여 미세한 거동을 측정할 수 있는 광섬유 정밀 계측 시스템을 개발하고, 실제 사면 현장에 시범 구축한 사례를 요약한 것이다. 영동고속도로 원주지사 관내에 있는 암반사면을 대상으로 총 6개의 센서를 변위발생 가능성이 높은 균열부에 설치하였다. 계측데이터의 수집과 전송을 위하여 사면 하단부에 컨트롤박스를 설치하고 데이터로거, 운용시스템, 유선네트워크 시스템을 구축하였다. 설정된 주기별로 취득되는 계측데이터는 우선적으로 현장에서 저장되고, 사무실에서도 원격으로 제어할 수 있는 시스템을 시범적으로 구축하여 운용중에 있다. 향후 계측 시스템의 센서부, 데이터로거, 운용프로그램 등을 추가 연구를 통하여 개선하고 현장에 반영하는 과정을 반복함으로써 사면 시설물의 유지관리에 최적의 시스템을 개발 및 구축하는 것이 본 연구의 최종적인 목표이다.

• Journal of the Korea Concrete Institute
• /
• v.26 no.4
• /
• pp.475-482
• /
• 2014

This paper presents the experimental results concentrically reinforced concrete tension members and compares cracking behavior of amorphous steel fiber and normal steel fiber reinforced concrete members. Two kind of steel fibers were included as a major experimental parameter together with the six cover thickness to bar diameter ratio ($c/d_b$). The presence of amorphous steel fibers effectively controlled the splitting cracks initation and propagation. In the amorphous steel fiber reinforced specimens, no splitting cracks were observed that becomes higher with cover thickness to bar diameter ratio is 2.0. Crack spacing of the each specimens reinforced with amorphous steel fibers and normal steel fibers becomes larger with the increase in cover thickness, and also measured maximum and average crack spacing are significantly smaller than current design code provision. Based on the measured crack spacings, a relationships for predicting the crack spacing is proposed using the measured average crack spacing in amorphous steel fiber reinforced concrete tension members.

• Journal of Korean Association for Spatial Structures
• /
• v.12 no.1
• /
• pp.51-58
• /
• 2012

A model for the nonlinear flexural analysis of extruded Engineered Cementitious Composite (ECC) panel reinforced concrete (RC) composite slab has been newly presented. From direct tensile test, ECC panel has been modeled to have the high-ductile tensile behavior after cracking. The developed model was compared with bending test results of two specimens, a conventional RC slab and a ECC panel RC composite slab. The predicted results were well patched with the experimental results, and the ECC panel RC composite slab system had advantages in crack control and improving flexural load-carrying capacity and deformation-capacity.